Over the last few years, research into superconducting magnets has progressed to the level where application of this technology is becoming a practical reality. As superconducting magnets typically must operate at cryogenic temperatures, e.g., down to about 77 K. and as low as 4 K., enclosures and devices used in the various applications of such magnets must necessarily also be able to withstand those extreme temperatures without failing. One example of such a material that has been used to prepare enclosures for one such application, e.g., coil cases for superconducting magnets, is stainless steel.
In the past, stainless steel alloys were formulated and developed so as to maximize their performance at high temperatures. This was primarily due to such alloys' known superior resistance to oxidation at those high temperatures as well as a lack of knowledge as to any applications for such alloys at cryogenic temperatures, e.g., superconducting magnet applications. Examples of such stainless steel alloys which are said to provide the aforesaid high temperature performance include those described in U.S. Pat. Nos. 2,696,433 and 2,839,391.
With the increasing interest in superconducting magnet applications, the evaluation of the properties of various stainless steel alloys at cryogenic temperatures has been undertaken with greater frequency. During such evaluations, it was found that at cryogenic temperatures, weld metals formed from stainless steel electrodes typically possessed a toughness which is significantly lower than that of the corresponding base metal alloy. Therefore, during the past fifteen years, certain studies have been undertaken in an effort to focus upon improving the physical properties of stainless steel welding metals, prepared from welding electrodes, in general at cryogenic temperatures.
One such study evaluated three welding electrodes (which will also be referred to herein as alloys) after they had been formed into a weld metal. This study determined that a ferrite-free 18Cr-16Ni-9Mn-0.14N electrode, when formed into a weld metal, exhibited the best combination of toughness (20 ft-lb Charpy V-notch (CVN) absorbed energy) and strength (124 KSI, 0.2% yield) at 77 K. The particular electrode which resulted in the preparation of a weld metal possessing the highest degree of toughness (28 ft-lb CVN) had a composition which can be generally described as 14Cr-20Ni-9Mn-0.08N (91 KSI, 0.2% yield) at 77 K. Fickett et al., Materials Studies for Magnetic Fusion Energy Applications at Low Temperatures--I, NBSIR 78-884 (April, 1978), pp. 173 & 174.
A second study evaluated three alloys and determined that the electrode from which a weld metal having the highest CVN (46 ft-lb) at 77 K. was prepared was an alloy comprised of 18Cr-16Ni-9Mn-0.08N (135 KSI, 2% yield strength). The yield strengths of three additional weld metals were evaluated at 4 K., with an alloy comprising 18Cr-14Ni-2Mo-0.07N providing a weld metal having the greatest strength (132 KSI) at 4 K., while exhibiting a toughness of 52 ft-lb CVN (after annealing) at 77 K. Fickett et al., Materials Studies for Magnetic Fusion Energy Applications at Low Temperatures--II, NBSIR 79-1609 (June 1979). Despite these advances however, neither of these studies offer any indication as to how the toughness and strength of weld metals prepared from stainless steel alloys could be improved, particularly if a weld metal prepared from such alloys was intended to be exposed to temperatures as low as 4 K.
Thus, and despite the availability of the aforesaid known alloys from which weld metals can be prepared, there remains a need for an alloy which, when formed into an electrode, enables one to prepare a welded stainless steel structure, wherein the weld metal formed from the electrode possesses a strength and toughness superior to that of existing weld metals, even when exposed to cryogenic temperatures, e.g., 77 K. and as low as 4 K.
The present invention provides such an improved alloy. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.